For an inkjet head formed by arranging a plurality of nozzles for ejecting ink drops in one direction, volumes of the ink drops ejected from respective nozzles are not always uniform. Thus, there are times when density unevenness is generated even in a case in which the same number of ink drops is ejected from each nozzle to print a solid image. Furthermore, there are times when a difference in level of density is generated at a border between an inkjet head and an inkjet head in a case in which a print area with a wide width is divided in a width direction and printing is carried out by a plurality of the inkjet heads which is arranged by matching an arrangement direction of the nozzles with the width direction.
The reason why the volumes of the ink drops ejected from the respective nozzles are not uniform is primarily attributable to unevenness in structure is generated in the inkjet head. For example, diameters of respective nozzles or volumes of pressure chambers separately communicating with the respective nozzles are not always uniform. In most cases, this unevenness in structure is caused by characteristics of a processing machine used at the time of manufacturing the inkjet head.
Conventionally, there is a technology for adjusting an ejection amount of ink drops for each nozzle by correcting a pulse width of a drive pulse signal applied to each of the actuators respectively corresponding to each of nozzles. The amount of the ink drops ejected from each nozzle can be homogenized using this technology. Correction data for correcting the pulse width for each nozzle has to be derived in order to homogenizing the amount. For example, 300 separate correction data must be derived for the inkjet head having 300 nozzles, which requires a cumbersome amount of time. JP2008-57781A discloses an image forming apparatus using a recording head in which discharge nozzles are two-dimensionally arranged with certain regularity is configured to form a reference image on the recording medium with a resolution corresponding to pitches of nozzles in the direction perpendicular to the direction according to which the relative movement of the medium and head is performed. Then, the reference image is scanned with a CCD line sensor, at a resolution lower than said resolution, and is used as reference image data. Then, an ink density distribution of nozzles is obtained based on the reference image data and the tendency of regularity of the ink density distribution. Then an ink density correction table for correcting the discharge amount of ink droplets by the discharge nozzles based on the ink density distribution whereby the discharge amount of the ink can be controlled. However, this method of correction is too complex.